Turbomachinery include impellers or rotors rotating at very high speeds. For gas turbine engines such as utilized in aerospace applications including commercial aircraft, it is important to sense excess motion of such a rotor as it is indicative of an impending engine failure such as may be due to a bearing failure, imbalance, foreign object damage, or other reasons. It is important that such failure be detected as rapidly as possible, and that the detecting mechanism be reliable and not subject to inadvertent operation.